Hi! After importing a ton of email from Outlook Express into Thunderbird, my friend noticed something odd when replying to one of the old Outlook Express imported messages. He's running Thunderbird 38.5.0 on Windows XP and the character encoding settings are the default Thunderbird settings. Outgoing mail is encoded as UTF-8 and incoming mail is encoded as Western (ISO-8859-1). Messages he reads are displayed with "Western" encoding. Cool.

When replying to a message originally received by Outlook Express, his sent message contains strange characters in it. Here's an example:

Windows 1252 is a superset of Western. Try changing the incoming mail character set to UTF-8 too. Hopefully that will solve the display problem. UTF-8 is a version of Unicode designed for backwards compatibility - the first 128 characters correspond one to one with ASCII.